# autumn equinox not till Sept. 26??? **URL:** \ **Category:** Factual Questions\ **Created:** [September 21, 2002, 8:10pm UTC](https://boards.straightdope.com/t/autumn-equinox-not-till-sept-26/129022 "2002-09-21T20:10:54Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![tclouie](https://avatars.discourse-cdn.com/v4/letter/t/838e76/32.png) [@tclouie](https://boards.straightdope.com/u/tclouie)\ **Post date:** [September 21, 2002, 8:10pm UTC](https://boards.straightdope.com/t/autumn-equinox-not-till-sept-26/129022/1 "2002-09-21T20:10:54Z")
According to the [U.S. Naval Observatory](http://aa.usno.navy.mil/data/docs/RS_OneDay.html), the hours of daylight will not equal the hours of night (that is, 12 and 12) until September 26! What gives? I thought the whole point of having an equinox was for day to equal dark. So what’s so special about today, September 21? Granted, this is information for Los Angeles, but shouldn’t it be the same all over the world? ---
**Author:** ![K364](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k364/32/5_2.png) [@K364](https://boards.straightdope.com/u/K364)\ **Post date:** [September 21, 2002, 8:17pm UTC](https://boards.straightdope.com/t/autumn-equinox-not-till-sept-26/129022/2 "2002-09-21T20:17:22Z")
If you dig a bit deeper… [http://aa.usno.navy.mil/faq/docs/RST\_defs.html#top](http://aa.usno.navy.mil/faq/docs/RST_defs.html#top) you will find that their definition of sunrise and sunset are not what you would expect. Specifically: > [@](#): > > For computational purposes, sunrise or sunset is defined to occur when the geometric zenith distance of center of the Sun is 90.8333 degrees. That is, the center of the Sun is geometrically 50 arcminutes below a horizontal plane. For an observer at sea level with a level, unobstructed horizon, under average atmospheric conditions, the upper limb of the Sun will then appear to be tangent to the horizon. The 50-arcminute geometric depression of the Sun’s center used for the computations is obtained by adding the average apparent radius of the Sun (16 arcminutes) to the average amount of atmospheric refraction at the horizon (34 arcminutes). ---
**Author:** ![sailor](https://avatars.discourse-cdn.com/v4/letter/s/a587f6/32.png) [@sailor](https://boards.straightdope.com/u/sailor)\ **Post date:** [September 21, 2002, 8:28pm UTC](https://boards.straightdope.com/t/autumn-equinox-not-till-sept-26/129022/3 "2002-09-21T20:28:23Z")
The equinox is when the sun crosses the equator and will be Sep 23 4h56.54m UTC. Due to the Equation of time the length of night and day are slighly affected but that is NOT what the equinox is. ---
**Author:** ![tclouie](https://avatars.discourse-cdn.com/v4/letter/t/838e76/32.png) [@tclouie](https://boards.straightdope.com/u/tclouie)\ **Post date:** [September 21, 2002, 8:47pm UTC](https://boards.straightdope.com/t/autumn-equinox-not-till-sept-26/129022/4 "2002-09-21T20:47:41Z")
Thanks for the info. BTW, I input some more dates for 2002 at the USNO site, and I found the following: 1. Sunrise and sunset were exactly 12 hours apart on March 16, 5 days before the “traditional” spring equinox. Just as the other 12-12 day, September 26, will be 5 days _after_ the “traditional” fall equinox. 2. June 21 and December 21 were, indeed, the longest and shortest days of the year. However, June 21 was part of a three-day block of days that all had the same hours of daylight. December 21 will be part of a _one-week_ block of days, all the same. ---
**Author:** ![sailor](https://avatars.discourse-cdn.com/v4/letter/s/a587f6/32.png) [@sailor](https://boards.straightdope.com/u/sailor)\ **Post date:** [September 21, 2002, 9:05pm UTC](https://boards.straightdope.com/t/autumn-equinox-not-till-sept-26/129022/5 "2002-09-21T21:05:29Z")
BTW, this question comes up regularly with the change of the seasons and you will find several threads discussing it in more detail